/Engine/Source/Runtime/Engine/Classes/Animation/AnimInstance.h
Plays an animation montage. Same as Montage_Play, but you can specify an AlphaBlend for Blend In settings.
float Montage_PlayWithBlendIn ( [UAnimMontage](API\Runtime\Engine\Animation\UAnimMontage) * MontageToPlay, const [FAlphaBlendArgs](API\Runtime\Engine\FAlphaBlendArgs) & BlendIn, float InPlayRate, EMontagePlayReturnType ReturnValueType, float InTimeToStartMontageAt, bool bStopAllMontages )